There are a number of manufacturing disciplines wherein selective film deposition may be beneficial. Perhaps the most prevalent of these is the manufacture of integrated circuits.
Manufacturing of integrated circuits is generally a procedure of forming thin films and layers of various materials on wafers of base semiconductor material, and selectively removing areas of such films to provide structures and circuitry. Doped silicon is a typical base wafer material. Chemical Vapor Deposition (CVD) and physical vapor deposition (PVD) are well known processes for depositing such thin films and layers in manufacture of integrated circuits and in other thin-film manufacturing. Sputtering is one example of a well-known and much-used PVD process.
As an example of CVD, polysilicon may be deposited from silane gas, SiH.sub.4. It is known, too, among many other processes, to deposit tungsten silicide from a mixture of gases including silane and a tungsten-bearing gas such as tungsten hexafluoride. Pure tungsten is also deposited on silicon wafers in the manufacture of integrated circuits, sometimes selectively and sometimes across the entire surface in a process known as blanket tungsten deposition.
One of the better known processes (and applications) of sputtering is deposition of aluminum electrical connections between solid state devices in manufacturing of integrated circuits. There are likewise many other applications for PVD generally and sputtering in particular.
In development of manufacturing techniques for integrated circuits and for many other products, a broad range of variations and combinations of both PVD and CVD techniques have been developed. For example, compounds, such as nitrides of metals, may be deposited by introducing a reactive gas, in this case nitrogen, into the typically inert gas (typically argon) used in the sputtering process to form a plasma.
A typical procedure for forming devices and interconnective circuitry on semiconductor wafers is one of coating the entire wafer with a film of a particular material, followed by patterning by techniques of photolithography, then etching to selectively remove unwanted regions of material, leaving selected patterns of the deposited material in place on the wafer surface. Patterning is also accomplished by means of application of photoresist material, curing of portions of the photoresist in a desired pattern, and removal of uncured photoresist material. The result of photoresist patterning is much the same as selective removal of other films; that is, a surface having patterned regions of more than one material.
Formation of conductive, connective circuitry between devices on a wafer is an example of a typical sequence of deposition, patterning, and selective removal of film regions. Coating device contacts in integrated circuits with materials providing high electrical conductivity and barrier characteristics to diffusion of materials is another example of a process typically accomplished by blanket coverage followed by patterning and selective removal, such as by etching.
There are manufacturing costs associated with every process step undertaken during wafer fabrication. It is also true that reliability and yield are adversely affected by additional processing. Any development that reduces the number and complexity of process steps is therefore generally beneficial.
One conceivable way to reduce the number of processing steps in semiconductor manufacturing is by developing processes for selective deposition. That is, if material can be deposited just on the regions of a wafer where the material is desired to remain, the additional steps of patterning and selective removal may be eliminated, along with the costs and reduced yields incurred by the additional steps.
Limited examples of selective deposition are known in the art. In the art of sputter deposition, for example, biased sputtering techniques have been demonstrated wherein the receiving substrate is biased, and so-called back sputtering of material from the receiving surface is accomplished during deposition. Under carefully controlled conditions, because sputtering rates may differ for a single species deposited on different material surfaces, active sputtering of the wafer surface may result in net material buildup on areas of one material, while areas of another material are held to no net gain in material thickness, or even a small net loss (sputter etched). A necessary condition to selectivity is that the region or regions desired to be coated are surfaces of a different material than the region or regions not to be coated.
Residual damage of silicon substrates and devices has been found to be a problem with biased sputtering, due to relatively high energy levels of bombarding species typically required to sputter etch materials.
Selective deposition has also been demonstrated in CVD processing, due primarily to preferential chemical activity at the surface of a particular material. For example, tungsten may be selectively deposited on silicon regions exposed through silicon dioxide, while there is no net deposition of tungsten on the regions of silicon dioxide. In this selective process, initial deposition of tungsten on silicon is through chemical reduction of WF.sub.6 by silicon. The oxide surface does not react chemically, so there is no net deposition on that surface. After initial deposition, and silicon is no longer available to the reactive gases as a reducing agent, an alternate agent must be added, typically hydrogen. In this case, as in the biased sputtering example described above, regions of different material are a necessary condition to selectivity.
A very important advantage expected for techniques of selective deposition is that, with a continuing trend to higher and higher device density, typical geometry has fallen below one micron. In the region of sub-micron geometry, patterning and selective removal become quite difficult. Selective deposition, however, is not limited by patterning considerations.
Presently known selective techniques are not without problems. These techniques, for example, are very limited in the range of materials that may be deposited, and often, as in the case of biased sputtering, unwanted damage to existing devices and structures occurs.
What is clearly needed is a means to selectively deposit a broad range of materials and/or phases of materials, including electrically insulating materials, with controllable selectivity, and with minimal damage to material and device structures.